Display devices, such as televisions, projectors, monitors, and camcorder viewfinders employ of variety of methods for generating colored images onto a viewing surface. One of such methods includes implementing an image processing unit (i.e., a processor) that is configured to control the color output of a display device using a spatial light modulator. The image processing unit sends instructions or control signals to the spatial light modulator, which modulates an incoming light beam in order to project a colored image onto a viewing surface. Common spatial light modulators include Digital Light Processing (“DLP”) chips and Liquid Crystal Based Panel Displays (“LCD”).
One of the more recent developments in display technology includes a light modulator containing an array of pixel elements defined by microelectromechanical system (MEMS) devices configured to act as tunable Fabry-Perot Interferometers (FPIs). In general, MEMS devices are microscopic mechanical devices fabricated using integrated circuit manufacturing technologies. In some cases, their mechanical structures include small flexures, similar to springs, which are capable of flexing components of the device over a limited range of motion.
In optical applications, MEMS devices can be configured into tunable Fabry-Perot Interferometers (FPIs), which are light filters that transmit incoming light within a particular bandwidth but reject light that is outside of that bandwidth. Generally speaking, an FPI pixel is defined by a set of reflective plates that are separated by a gap. Incoming incident light from a light source reflects back and forth between the reflective plates. The gap between the plates provides interference to the incident light, which changes the light's incoming wavelength. Therefore, the wavelength of the resultant light that is transmitted from the FPI pixel is dependent on the distance between the reflective plates (i.e., the optical gap). In other words, the output color for each FPI pixel can be controlled by adjusting the distance between the reflective plates.
In one embodiment, the reflective plates of an MEMS FPI pixel are reflective capacitive plates that form a capacitor with a top movable plate having flexures, and a bottom fixed plate. The position of the plates, which defines the amount of gap therebetween, can be controlled by applying a voltage to each of the plates. The applied voltage creates an electrostatic field, which pulls the plates together. However, the flexures, which are spring-like structures that allow the top plate to move into position, exert a spring force that opposes the electrostatic field generated by the applied voltage. When the electrostatic field and the spring force are properly balanced, a stable optical gap is achieved that can be represented by the capacitance between the plates (i.e., gap capacitance). In other words, for a given optical gap (i.e., color) there is a corresponding gap capacitance that is determined based on a particular applied voltage. Therefore, the color output of an FPI pixel can be controlled by applying a voltage that will produce an expected gap capacitance.
Unfortunately, the expected gap capacitance changes over time due to a gradual change in the spring constant of the flexure regions. The change in spring constant affects the balance between the spring force and the electrostatic field when a voltage is applied. As a result, an applied voltage will produce a gap capacitance that varies over time. For this reason, it is advantageous to continually measure the gap capacitance and calibrate the relationship between the gap capacitance and the applied voltage.
Known methods for measuring capacitance in an FPI device include mechanically holding the FPI device in position with a probe to measure the capacitance. This method can be a slow and challenging process which requires costly test equipment. In addition, the process can only be applied once at the manufacturing stage prior to shipping. In this case, there is no way to calibrate the display device during regular use.
One known calibration method involves directly measuring the color using optical sensors. This method is beneficial in that the optical sensor interfaces directly with the light path, which provides an extremely accurate measurement of the light wavelength (i.e., color). However, measuring color using this method requires an optical sensor that can contribute significantly to the overall cost of the display.
The embodiments described hereinafter were developed in light of these and other drawbacks associated with measuring and calibrating a light modulator employing FPI pixels.